Twins are formed when either one fertilized egg splits into two or two eggs get fertilized at the same time, producing one pregnancy, but two babies.

There are two main categories of multiples—Fraternal and Identical. Similarity in looks does NOT determine whether twins are identical versus fraternal.

Identical, or Monozygotic Twins occur when a single fertilized egg splits into two embryos around the time the fertilized egg is becoming implanted in the womb (usually between the fourth and 12th day after fertilization). Monozygotic twins make up 1/3rd of all twins. The two embryos develop into fetuses sharing the same womb. When one egg is fertilized by one sperm cell, and then divides and separates, two identical cells will result. If the zygote splits very early (in the first 2 days after fertilization) they may develop separate placentas and separate sacs . In about 1% of identical twins the splitting occurs late enough to result in both a shared placenta and a shared sac. Finally, the zygote may split extremely late, resulting in conjoined twins.

Fraternal Or Dizygotic Twins are the result of two separate eggs (ovum) becoming fertilized by two separate sperm, resulting in two completely distinct pregnancies in the womb at the same time. They are known as non-identical or dizygotic twins. Dizygotic twins account for about 2/3rds of all twins.

Mirror twins

Some monozygotic twins are known as "mirror twins" or mirror image twins. These are identical twins with opposite features, that is, one may be right-handed and the other left, mirrored hair curls, etc. This condition is comparatively rare in humans: about 25% of all identical twins are mirrored to some degree, but in the vast majority of these cases it is a very minor feature - such as mirrored finger prints. Mirroring results from a late split of the fertilized egg at around 9-12 days. One mirror may have situs inversus, where some or all of the organs are on the opposite side of the body, such as the left ventricle of the heart on the right (dextrocardia). Such conditions are usually associated with a higher incidence of other birth defects, and are very rare.

Conjoined Twins

Conjoined twins (or "Siamese twins") are monozygotic twins whose bodies are joined together at birth. This occurs where the single zygote of identical twins fails to separate completely, and the zygote starts to split after day 13 following fertilization. This condition occurs in about 1 in 50,000 human pregnancies. Most conjoined twins are now evaluated for surgery to attempt to separate them into separate functional bodies. The degree of difficulty rises if a vital organ or structure is shared between twins, such as brain, heart or liver.

 Still some questions in mind?

How to identify the twins are identical Scientifically?

DNA test is the most accurate of all. Other methods are Blood test or Examination of Placenta. but these are less accurate as compared to DNA.

All the identical twins has to be same-sex i.e., Boy/Boy or Girl/Girl. Until very recently, we know that there are certain rare instances when identical twins can actually be different-sex individuals; when this has occurred and been documented, the twins are genetically identical in every way other than a slight chromosomal difference.
But make no mistake, not every set of boy/boy or girl/girl twins is identical. In fact, many same-sex sets of twins are fraternal twins. Very often, same-sex twins look very different, indeed, sometimes right from birth. They all have different-colour hair and eyes, are of different height and have different physical builds, and display very different mannerisms.
When it comes to triplets, quads, quints, and even larger higher-order multiples, they are the combination of one pair of identical & one pair of fraternal set.

(*Source Wikipedia  & Twinsmagazine.com)